The present invention relates to an electromagnetic steel sheet having excellent magnetic properties, especially within a frequency range higher than commercial frequency, and to a method of making the same.
Silicon steel is known for its excellent soft magnetic properties. Si steel essentially having an Si content of 3.5% by weight or less is usually employed as iron cores in power-frequency motors, transformers, etc. However, when such Si steel is used within a frequency range of 1 kHz or more, that is higher than commercial frequency, the iron loss caused by eddy currents is excessive. Therefore, Si steels of that type are disadvantageous for use in iron cores in many electric appliances.
With the recent tendency toward small-sized and high-performance electric appliances, there is an increasing demand for high-performance motors, high-frequency transformers, etc. They demand materials having small iron loss.
Within an extremely high frequency range (100 kHz or higher), the eddy-current loss in steel sheets is enormous. Therefore, for use in such an extremely high frequency range, ferrite has heretofore been employed as iron cores, even though its magnetic flux density is low.
In this connection, an increase of Si content of steel brings about an increase in its electric resistance, thereby resulting in reduction of the eddy currents induced in the steel. Therefore, the iron loss of such high-Si steel is favorably reduced within a frequency range higher than commercial frequency. However, Si steel having an Si content larger than 3.5% by weight is extremely hard and brittle, and its workability is poor. Therefore, it is extremely difficult to produce Si steel sheets of that type by rolling. In particular, the workability of Si steel having an Si content greater than 5.0% by weight is so poor that it cannot be subjected to cold rolling, or even to warm rolling.
Regarding the technique directed to the industrial-scale production of steel sheets having an Si content of around 6.5% by weight, hot rolling at a low temperature and under a high reduction, is disclosed in Japanese Patent Application Laid-Open (JP-A) Sho-61-166923, and a method is disclosed for processing steel for Si diffusion penetration, in JP-A Sho-62-227078.
However, the technique disclosed in JP-A Sho-61-166923 requires delicate control of the rolled steel texture for seemingly reducing the brittleness of the steel. Therefore, in the disclosed method, the steel must be strictly controlled in production, and it is difficult to stably produce steel sheets on an industrial scale according to the method. On the other hand, the technique disclosed in JP-A Sho-62-227078 requires specific diffusion coating with Si, and is therefore extremely disadvantageous for industrial production of steel sheets, as being too expensive.
An increase of the Si content in steel up to 6.5% by weight can bring about an increase of specific resistivity to only the level of at most 80 xcexcxcexa9xc2x7cm or so. In particular, for steel sheets having an Si content not larger than 3.5% by weight, that could be produced in ordinary industrial rolling methods, the sheets could have a specific resistivity of up to the level of 50 xcexcxcexa9xc2x7cm or so. In other words, a further increase of the electric resistance of steel to be attained by Si addition only is limited, and the mere addition of Si to steel is insufficient for obtaining steel having good high-frequency magnetic properties.
In addition, Si steel is said to be further problematic in use for iron cores, as having poor corrosion resistance.
On the other hand, it is known that Al is effective for increasing electric resistance of steel, like Si. Al does not so greatly reduce the workability of steel. Therefore, substituting for a part of Si in steel with Al would seem to be effective for improving the workability of Si steel while increasing its electric resistance. For example, steel containing 3% by weight of Si and 0.7% by weight of Al has better workability than Al-free steel containing 3.7% by weight of Si. Yet both have nearly the same magnetic properties. However, such Al-containing steel is disadvantageous in that Al is more expensive than Si, and that Al causes significant reduction of magnetic flux density of the Al-containing steel. For another type of Al-containing steel having an Si content of not smaller than 3% by weight, in which the total of Si and Al is not smaller than 4% by weight, its workability is also poor, and cold rolling of the steel is impossible. For still another type of Al-containing steel in which the total of Si and Al is more than 6% by weight, its workability is so poor that even warm rolling of the steel is difficult. In short, steel sheets containing Si and Al to such a degree that the total of Si and Al therein is less than 4% by weight could be produced on an industrial scale, but without practical benefit because their specific resistivity could not be over 60 xcexcxcexa9xc2x7cm.
Even if the amounts of Si and Al added to steel are increased enough to reduce the iron loss in the resulting Si-Al steel within a high frequency range, the essential workability of the steel would not be improved, the corrosion resistance of the steel would be poor, and that the production costs for the steel would be high.
For improving the corrosion resistance of Si steel, a method is disclosed comprising adding a predetermined amount of Cr to the steel (JP-A Sho-52-24117 and JP-A Sho-61-27352). As in those references, addition of Cr to Si steel is known. However, the magnetic properties of the steel disclosed in those publications are still the same as those of ordinary Cr-free Si steel. The magnetic properties of the steel are not improved to a significant degree by the addition of Cr.
An object of the present invention is to provide electromagnetic steel sheets which have excellent workability, good high-frequency magnetic properties with high specific resistivity, and even good corrosion resistance, all achieved at low cost. Steel sheets of improved workability could be worked into thinner sheets having even more improved high-frequency magnetic properties.
We have made a novel discovery that, for ensuring good workability of Si steel and Sixe2x80x94Al steel, under certain conditions, adding Cr to Si steel or Sixe2x80x94Al steel is surprisingly effective for improving the workability of the steel.
In this connection, it has heretofore been considered that addition of an increased amount of Cr to steel reduces the workability of the resulting steel. As opposed to this, however, we have found that, even in Sixe2x80x94Al steel having an Si content of at least 3% by weight and an Al content of at least 1% by weight, the presence of a specific amount of Cr improves the workability of the steel when the (C+N) content of the steel is reduced to a critical level.
In addition, we have further discovered that even Cr-containing Si steel or Cr-containing Sixe2x80x94Al steel having a smaller Si content and a smaller Al content and having a specific resistivity of at least 60 xcexcxcexa9xc2x7cm can have much improved workability than Cr-free Si steel or Cr-free Sixe2x80x94Al steel having the same degree of specific resistivity, if its (C+N) content is reduced to the requisite level.
Moreover, we have found that the presence of Cr along with Si and Al in steel brings about a synergistic effect in increasing the electric resistance of the steel.
Based on these findings, we have reached the result that the iron loss in such Cr-containing steel, especially within the high frequency range, is reduced much more than Si steel, Al steel or even Sixe2x80x94Al steel containing Si and/or Al but not Cr. In addition, the corrosion resistance of the Cr-added Si steel is significantly improved, more than that of conventional Cr-free Si steel.
This invention provides an electromagnetic steel sheet with excellent high-frequency magnetic properties. It contains Cr in an amount of from about 1.5 to 20% by weight, and Si in an amount of from about 2.5 to 10% by weight, while having a maximum total (C+N) content of about 100 ppm by weight, and which has a specific resistivity of at least about 60 xcexcxcexa9xc2x7cm. The steel sheet may contain Al in a maximum amount of about 5% by weight, and/or one or two elements selected from Mn and P, each in a maximum amount of about 1% by weight.
Preferably, the steel sheet has a thickness of from about 0.01 to 0.4 mm.
The invention also provides a method for producing electromagnetic steel sheets with excellent high-frequency magnetic properties, which comprises hot rolling a steel slab containing Cr in an amount of from about 1.5 to 20% by weight, and Si in an amount of from about 2.5 to 10% by weight and having a maximum (C+N) content of about 100 ppm by weight, into sheets having a maximum thickness of about 3 mm.
Experiments and data are now described for the purpose of full explanation. The Examples are not intended to define or to limit the scope of the invention, which is defined in the appended claims.
Using raw materials Fe, Cr, Si and Al, all having a purity of at least 99.99%, we prepared Cr-added 4.5 wt. % Si-2 wt. % Al steel ingots having a Cr content of 0, 2, 4 or 12% by weight, in a small-sized, high-vacuum (1xc3x9710-4 Torr) melting furnace. The weight of each ingot was 10 kg. Regarding the impurity contents of the steel ingots, the C content was from 5 to 8 ppm by weight, the P content was from 3 to 5 ppm by weight, the S content was from 2 to 3 ppm by weight, the N content was from 12 to 18 ppm by weight, the O content was from 11 to 15 ppm by weight, and the (C+N) content was from 18 to 22 ppm by weight. Each steel ingot was cut into slabs having a thickness of 60 mm, and rolled into sheets having a thickness of 3.2 mm after heating at 1100xc2x0 C.
From each steel sheet we cut out Charpy test pieces having a thickness of 2.5 mm, a width of 10 mm and a length of 55 mm. Each test piece was V-notched to a length of 2 mm. The lengthwise direction of each test piece was parallel to the rolling direction thereof. All test pieces were subjected to a Charpy test at different temperatures up to 250xc2x0 C., and the area percent brittle fracture of each test piece at different temperatures was obtained. From the data obtained, the temperature at which the area percent brittle fracture of the test piece shall be 50% was obtained through interpolation. The temperature at which the area percent brittle fracture of a steel sheet is 50% is referred to as the ductility-brittleness transition temperature of the steel sheet; this is known as an index of the toughness of steel. The workability of steel may be evaluated on the basis of this transition temperature. Steel having a lower transition temperature has higher toughness and better workability. The influence of the Cr content of steel on the transition temperature thereof is shown in Table 1.
Unexpectedly, the transition temperature of steel lowered with the increase in the Cr content thereof, as in Table 1. This means that the workability of steel increased with an increase of the Cr content thereof. In addition, it was verified that Cr added to steel in an amount of at least 2% by weight exhibited a workability improving effect, and that the workability improving effect of Cr addition was saturated even though more than 20% by weight of Cr was added to steel. Steel having a transition temperature of not higher than 200xc2x0 C. could be subjected to ordinary warm rolling at around 300xc2x0 C. or so. Steel having a transition temperature of not higher than 100xc2x0 C. could be, after having been first heated at a temperature not higher than 200xc2x0 C., subjected to ordinary cold rolling, and is therefore further advantageous in its industrial process.
In the next experiment, we prepared ingots of 4 wt. % Cr-4.5 wt. % Si-2 wt. % Al steel in the same manner as previously, to which, however, we added a matrix alloy of Fe-5 wt. % C and iron nitride so as to control the C content and the N content of those ingots. The steel sheets thus prepared each had a different (C+N) content, and these were subjected to the same Charpy test as previously. The test data obtained are shown in Table 2.
As in Table 2, the workability of steel samples having a (C+N) content of about 100 ppm by weight or lower was significantly improved. Steel having a (C+N) content of about 100 ppm by weight or lower could be subjected to ordinary warm rolling.
Next, of the hot-rolled sheet samples, those of 4 wt. % Cr-4.5 wt. % Si-2 wt. % Al steel having a (C+N) content of 19 ppm by weight, and comparative samples of 6 wt. % Si steel (of which the (C+N) content was 19 ppm by weight) were warm-rolled into thinner sheet samples having a thickness of 0.2 mm, which were then annealed in a hydrogen atmosphere at 1200xc2x0 C. for 60 minutes. The thus-annealed samples were tested to measure their specific resistivity and magnetic properties. Precisely, the hot-rolled sheet samples of 4 wt. % Cr-4.5 wt. % Si-2 wt. % Al steel were heated at 300xc2x0 C. and subjected to ordinary warm rolling. However, the comparative samples of 6 wt. % Si steel were too brittle, and could not be subjected to ordinary warm rolling. Therefore, the comparative samples of hot-rolled sheets were heated at 450xc2x0 C., and rolled into sheets having a thickness of 0.2 mm after having been specifically re-heated in every rolling pass. The thus-rolled sheets of 4 wt. % Cr-4.5 wt. % Si-2 wt. % Al steel had a specific resistivity of 120 xcexcxcexa9xc2x7cm, which was much higher than the specific resistivity, 81 xcexcxcexa9xc2x7cm of the rolled sheets of 6 wt. % Si steel. The iron loss in the sheets of 4 wt. % Cr-4.5 wt. % Si-2 wt. % Al steel at a frequency of 10 kHz and a magnetic flux density of 0.1 T was 15 W/kg, which was much smaller than the iron loss of 18 W/kg in the sheets of 6 wt. % Si steel.
The present invention is based not only upon the specifically-selected additive components to steel, but upon the purity of the steel.
The reasons for the numerical limitations of the constituent components of steel of the invention are described below.
Cr added to steel acts to greatly increase the electric resistance of steel, owing to the synergistic effect of Si and Al as combined with Cr, thereby reducing the iron loss in the steel within a high frequency range. In addition, Cr is a basic component for improving the corrosion resistance of steel. In particular, even to steel containing Si in an amount of at least 3.5% by weight or containing Si in an amount of at least 3% by weight along with Al in an amount larger than 1% by weight, addition of Cr is extremely effective for improving the workability of the steel, thereby making it possible to subject the steel to ordinary warm rolling. From the viewpoint of improving the workability of steel, Cr shall be added to steel in an amount of at least about 2% by weight. If the Si content and the Al content of steel are less than the ranges noted above, the workability of the steel can be ensured even though a smaller amount of Cr below about 2% by weight is added to the steel. However, in order to ensure the workability improving effect of the Cr addition and to make the steel alloy have a specific resistivity of at least about 60 xcexcxcexa9xc2x7cm, addition of Cr in an amount at least about 1.5% by weight is indispensable. On the other hand, if the amount of Cr added is larger than about 20% by weight, the workability improving effect of Cr addition becomes saturated, and addition of such a large amount of Cr causes increase of the production costs. For these reasons, the Cr content of the steel sheet of the invention is defined to fall between about 1.5 and 20% by weight, but preferably between about 2 and 10% by weight, more preferably between about 3 and 7% by weight.
Si addition to steel acts to greatly increase the electric resistance of steel, owing to the synergistic effect of Cr as combined with Si, thereby reducing the iron loss in the steel within a high frequency range. If the amount of Si added to steel is smaller than about 2.5% by weight, the steel does not have an increased specific resistivity of at least about 60 xcexcxcexa9xc2x7cm without so much lowering its magnetic flux density, even when Cr and Al are added to the steel along with Si. On the other hand, however, if the amount of Si added is larger than about 10% by weight, the workability of the steel cannot be ensured to such a degree that the steel could be subjected to ordinary warm rolling even when Cr is added to the steel along with Si. For these reasons, the Si content of the steel sheet of the invention is defined to fall between about 2.5 and 10% by weight, but preferably between about 3 and 7% by weight, more preferably between about 3.5 and 5% by weight.
Like Si, Al is effective for greatly increasing the electric resistance of steel, owing to the synergistic effect of Cr as combined with Al, thereby reducing the iron loss in the steel within a high frequency range. Therefore, in the invention, Al may be optionally added to the steel sheet. However, adding Al in an amount of larger than about 5% by weight causes a significant increase in the production costs. In addition, if too much Al is added to the steel sheet of the invention having an Si content of about 2.5% by weight or more, the workability of the steel sheet cannot be ensured to such a degree that the steel sheet could be subjected to ordinary warm rolling even when Cr is added to the steel sheet. For these reasons, therefore, the maximum Al content of the steel sheet of the invention should be about 5% by weight. For improving the deoxidizability of the steel and promoting the grain growth in the steel sheet, Al must, be added to the steel sheet in an amount of from about 0.005 to 0.3% by weight or so. In addition, in order to positively use Al for increasing the electric resistance of the steel sheet of the invention having an Si content of about 2.5% by weight or more, adding Al to the steel sheet in an amount of smaller than about 0.5% by weight is ineffective. Therefore, the amount of Al to be added to the steel sheet of the invention is preferably from about 0.005 to 5% by weight, more preferably from about 0.5 to 3% by weight.
C and N, if present, lower the toughness of Crxe2x80x94Si steel. Therefore, their percentages must be as small as possible. In the steel sheet of the invention of which the Cr content, the Si content and the Al content are within the ranges defined above, the maximum total amount of C and N must be reduced to about 100 ppm by weight in order to ensure good workability of the steel sheet. Preferably, the total amount of C and N is at most about 60 ppm by weight, more preferably at most about 30 ppm by weight. For individual cases of C and N, preferably, the maximum C content is about 30 ppm by weight and the maximum N content is about 80 ppm by weight, more preferably, the maximum C content is about 10 ppm by weight and the maximum N content is about 20 ppm by weight.
The amount of the other impurities except C and N is not specifically defined. However, the preferred ranges of the other impurities are as follows: maximum S is about 20 ppm by weight, preferably about 10 ppm by weight, more preferably about 5 ppm by weight. Maximum O is about 50 ppm by weight, preferably about 30 ppm by weight, more preferably about 15 ppm by weight. The maximum total amount of the impurities C+S+N+O is preferably about 120 ppm by weight, more preferably about 50 ppm by weight.
It is known that Mn and P, if added to Crxe2x80x94Si steel, further increase the electric resistance of the steel. Adding those components to the steel of the invention attains further reduction in the iron loss in the steel, without interfering with the workability of the steel. Therefore, in the present invention, one or two elements selected from Mn and P may be added to steel. However, adding too much Mn and P to steel substantially increases the production costs. Therefore, the maximum amount of those components to be added shall be about 1% by weight each, more preferably about 0.5% by weight each.
In the present invention, any conventional alloy components may be further added to steel for the purpose of further improving the magnetic properties, the corrosion resistance and the workability of the steel, as not interfering with the toughness of the steel. Some typical examples of such additional components will be mentioned below.
A maximum Ni of about 5% by weight can be a corrosion resistance-improving component. In addition, this lowers the ductility-brittleness transition temperature of steel, while improving the workability thereof. In addition, as facilitating easy creation of fine grains in steel, Ni tends to reduce the eddy-current loss in steel, while reducing the high-frequency iron loss therein. Maximum Cu of about 1% by weight may exhibit the same effect as Ni. Maximum Mo and W of about 5% by weight improve the corrosion resistance of steel. La, V and Nb of maximum about 1% by weight, and Ti, Y and Zr of maximum about 0.1% by weight, and even B of maximum about 0.1% by weight increase the toughness of steel, while improving workability. A maximum Co of about 5% by weight increases the magnetic flux density of steel, and is additionally effective for reducing the iron loss in steel. Sb and Sn of maximum about 0.1% by weight improve the texture of steel, and are additionally effective for reducing the iron loss in steel.
A method of producing steel sheet of this invention is described below.
In producing a melt of Crxe2x80x94Si steel or Crxe2x80x94Sixe2x80x94Al steel of the invention, it is desirable to use, as starting materials, high-purity electrolytic iron, electrolytic chromium, metal Si and metal Al, all having a purity of at least about 99.9% by weight. Where Mn and P are added to the steel, it is also desirable to use high-purity materials of those elements. Where the steel melt is produced in a converter, it is necessary that the steel melt produced is fully refined to have a predetermined purity and that the steel melt is not contaminated in the post-treating steps. Apart from a converter, the steel melt may be produced, for example, in a high-vacuum melting furnace (having a reduced pressure of not higher than 10xe2x88x923 Torr).
The steel ingots thus produced in the manner noted above are hot-rolled into sheets as thin as possible, which have good rollability in the next cold-rolling or warm-rolling step. For steel sheets having an Fexe2x80x94Crxe2x80x94Si alloy composition of the invention, it is believed that the toughness of the surface part of the hot-rolled sheets is higher than that of the center part thereof, and therefore the total workability become better. In order to make the steel sheets of the invention have better rollability, it is desirable that the maximum thickness of the hot-rolled sheets is about 3 mm, preferably about 2.5 mm, more preferably about 1.5 mm.
Since the workability of the hot-rolled sheets of the invention is good, the sheets can be further warm-rolled or cold-rolled to have a maximum reduced thickness of about 0.4 mm. It has heretofore been known that, in ordinary steel sheets having reduced thickness, the eddy-current loss is advantageously reduced especially within a high frequency range, and the iron loss is thereby reduced. However, conventional steel sheets having a high specific resistivity have poor workability and, when rolled in an ordinary manner, they can be thinned to have a reduced thickness of at least about 0.5 mm or so. In addition, it has heretofore been considered that, if conventional steel sheets are merely thinned to have a reduced thickness, the hysteresis loss in the thinned sheets is rather increased and therefore the iron loss therein could not be reduced to a satisfactory degree. As opposed to the conventional knowledge, however, the iron loss in steel sheets having the specific alloying composition and having the specific purity of the present invention, can be lowered to a satisfactory degree even within the high frequency range, merely by reducing the thickness of the sheets. In order to obtain the intended results through thickness reduction in steel sheets, it is effective to make the steel sheets have a maximum reduced thickness of about 0.4 mm. However, thickness reduction to smaller than about 0.01 mm would be disadvantageous in view of high production costs and of the current technical level. Therefore, in the present invention, the thickness of the steel sheets may be defined to fall between about 0.01 and 0.4 mm, preferably between about 0.03 and 0.35 mm.
Since the workability of the steel material of the invention is good, the invention does not require any additional treatment for ensuring and improving the workability of the steel sheets, for example, by annealing the hot-rolled sheets, or by subjecting them to intermediate annealing in the course of cold rolling or warm rolling, being different from the conventional methods for producing steel sheets. Therefore, for improving working capacity, saving energy consumption and reducing production costs in the invention, annealing of hot-rolled sheets and even intermediate annealing of cold-rolled or warm-rolled sheets can be omitted.
For annealing and surface-treating the sheets of the invention, the same steps as those for ordinary electromagnetic steel sheets and electromagnetic stainless steel sheets apply.